Sunday, October 18, 2009

So the design at the moment for the downconverter looks like it'll use the MC145151 PLL and the SA620 LNA/Mixer/VCO. At first, I figured I'd use the MC12080 divide-by-80 prescaler. With a 12.8 MHz crystal and a divide by 1024 reference divisor, that's a channelization of 1 MHz. And that's fine for an ATV downconverter.

But what if we altered the design a bit to take advantage of the fractional capabilities of the a dual modulus PLL chip, like the MC145152?

If we used that same 12.8 MHz crystal and divide by 512, we'd have a 25 kHz reference frequency. With a divide-by-64/65 prescaler (like the MC12054A), we'd be able to achieve the same result, but with a 25 kHz channel step.

A PLL that can drive a dual modulus prescaler has two counters. One of them is the actual output into the phase comparator, the other causes a digital output to change state during each count cycle. That signal changes the prescaler from a divide by P+1 to a divide by P.

Let's say that we set the main counter value to N and the prescaler change counter to A and the lower of the two prescaler values to P. What we'd then wind up with is dividing the VCO output by P*N+A. If the reference frequency is R, then the output frequency will be R*(PN+A). So for 848 MHz, if P is 64, and R is 25 kHz, N would be set to 530 and A to 0. Set A to 1 and the output would instead be 848.025 MHz. For 849 MHz (which puts 909-915 down to TV channel 3), you set N to 530 and A to 40.

In general, for a desired frequency F, you set the N to the integral quotient of (F/R) divided by P. You then set A to (F/R) mod P. Stated another way, you're building a fraction of N + A/P, which winds up being equal to F/(P*R).

Of course, this does mean that the design will need more DIP switches. The MC145152 has 6 bits for the A value and 10 bits for the N value. That's two banks of 8 switches. You could fix the top 4 bits of N to 1000 - limiting N from 512 to 575. The resulting frequency range would be 819.2 MHz to 921.6 MHz - more than enough for our purposes. That's a total of 12 switches - two banks of 6.

So a 12.8 MHz crystal, an MC145152 PLL, and an MC12054A prescaler and an SA620 LNA/VCO/Mixer.

Now the big problem is either finding inventory on these parts somewhere or finding equivalents.